1. Field of the Invention
This invention relates to a liquid detergent which possesses the property of substantially no color fading or reduced color fading, and which comprises as the principal effective ingredient an anionic sulfate- or sulfonate-type organic surfactant.
2. Description of the Prior Art
Anionic synthetic non-soap surfactants having a sulfate or sulfonate group are most frequently and most generally used as effective detersive ingredients of liquid detergents, such as dish washing detergents, shampoos, heavy duty liquid detergents, household detergents and the like, and they are employed in various forms.
In order to increase their commercial value, these liquid detergents are colored with various colorants. In the case of liquid detergents comprising an anionic sulfate- or sulfonate-type organic surfactant, however, there occurs extreme color fading of the detergent composition when it is exposed to sunlight for a long time or it is stored at relatively high temperatures for a long time. It is highly desirable to prevent color fading in these liquid detergents. Further, perfumes are generally incorporated in these liquid detergents, and when the detergents are stored for a long time (especially at high temperatures or in summer), these perfumes deteriorate or their fragrances change.
As a result of our research on color fading of dyes in solutions containing anionic organic surfactants containing a sulfate or a sulfonate group in the molecule, we found that minute amounts of by-products are formed during the sulfation or sulfonation reaction employed to prepare the surfactants and that the presence of these by-products in the liquid detergent composition has a great influence on color fading of the dyes or colorants therein. It was also found that if these by-products are inactivated chemically, the color fading can be considerably reduced or prevented. Based on these findings, we have now completed this invention. This is also true with respect to perfumes. Thus, by chemical inactivation of such by-products, degradation or deterioration of perfumes can be effectively reduced or prevented.
Anionic organic surfactants having a sulfate or sulfonate group in the molecule are synthesized by using highly reactive chemicals such as chlorosulfonic acid, SO.sub.3 gas and propane sultone. Accordingly, in addition to the main reactions that occur, such as sulfation and sulfonation, various complicated side reactions also take place simultaneously during the synthesis process, and hence, various by-products are formed. In fact, when the characteristics of surfactants of this type are measured, peroxide values (hereinafter referred to as "POV") of a certain level are inevitably noted. Especially when SO.sub.3 gas is employed as a sulfating or sulfonating agent, a high POV is observed. Accordingly, the occurrence of side reactions which produce peroxide compounds is expected in sulfation or sulfonation reactions. Although the chemical structure of the compound or compounds that provide such POV is unknown, it is found that the stability of colorants and the stability of perfumes are greatly degraded by such POV-providing compounds. More specifically, it was found that when a reducing agent, such as sodium nitrite, sodium sulfite and sodium thiosulfate, is added to an aqueous solution of an anionic surfactant of this type in an amount that precisely corresponds to the POV value thereof, the POV-providing compounds destroyed, and the stability of the colorants and perfumes is greatly improved. However, if such reducing agent is incorporated in an amount larger than the amount equimolar to the POV-providing compounds, the colorant or perfume is deteriorated and it sometimes happens that the entire solution is drastically discolored or a bad smell is generated. Accordingly, in the method in which a reducing agent is added to the detergent composition in order to prevent color fading, it is important to determine in advance the precise amount of the reducing agent that needs to be added to remove the POV-providing compounds. However, the POV differs greatly among various lots of sulfate- or sulfonate-type anionic organic surfactants, and therefore, this method requires very complex analysis procedures for measurement of the POV of each lot and determination of the amount of the reducing agent that must be added to it to remove the POV-providing compounds. Therefore, it is difficult to employ such method in practical industrial use.